FORMATION OF ROOTS ON THE SHADED SIDE OF SHOOTS. 



5^3 



So long ago as 1863 I described various examples showing that the formation 

 of roots upon shoot-axes by darkening them, may be induced at spots where 

 otherwise no roots arise: here however I need only mention TropcBolum majus 

 (Indian Cress). If one of the long thin leafy shoots is bent down as it were into 

 a depression of the surrounding garden soil, so as not to break it off and so that 

 an apical portion of the shoot, some 10-12 cm. long, remains upright, and can be 

 surrounded with damp soil, new roots arise after a few days all around the 

 vertical piece of shoot-axis, while the older piece of the same shoot-axis, lying 

 horizontally and likewise covered 

 with earth, developes roots exclu- 

 sively on its lower side. This ex- 

 periment shows not only that the 

 exclusion of light generally induces 

 the development of roots on the 

 shoot-axis, but the simultaneous 

 influence of gravitation is here very 

 clearly apparent alsa: the horizontally 

 placed shoot-axis forms roots only 

 on the lower side, the vertically up- 

 right one on all sides. Of course 

 in this experiment the growth of 

 the new roots is promoted by the 

 moisture of the surrounding soil. 

 The cause of their first production, 

 however, by no means resides in that, 

 since the same result is obtained if a 

 leaf-shoot of Tropceolum is directed 

 into the dark cavity of an opaque 

 box without separating it from its 

 parent plant, only in this case the 

 new roots remain very short unless 

 the air in the box is in some way 

 kept moist artificially. 



These examples refer to the 

 origin of new growing - points of 

 roots on shoot-axes ; but, in many 

 cases, roots may arise in the trans- 

 lucent tissue of sub-aerial shoot-axes, 



that is under the influence of light, although it may be feeble. Thus, in the 

 creeping or climbing shoots of the Gourd-plant we find a rudimentary root to 

 the right and left of each leaf, and similarly on those parts of the erect stem of the 

 Maize plant which are about 10-12 cm. above the surface of the soil: here 

 the roots come forth in large numbers in a crown above the insertion of the leaf. 



FIG. 339. — Ivy-shoot [Hednrti Helix\. A has been illuminated on the 

 dorsal side for several days, B has undergone similar treatment on the 

 ventral side. C a later stage of B. 



several times in my researches, ' Vber den Einfluss des Tageslichtes auf Neubildung und Entfaltung 

 verschiedener Pfianzenorgane^ (Bot. Zeigt. 1863, supplement r. and the same, 1865, pp. ii^j &c.). 



